A thermal inkjet printer typically includes one or more reciprocating print cartridges in which small drops of ink are formed and ejected towards a medium upon which it is desired to place alphanumeric characters, graphics, or images. Such cartridges include a printhead having an orifice plate that has a plurality of small nozzles through which the ink drops are ejected. Adjacent to the nozzles are ink firing chambers, in which ink resides prior to ejection through the nozzle. Ink is supplied to the ink-firing chambers through ink channels that are in fluid communication with an ink supply, which may be contained in a reservoir portion of the print cartridge or in a separate ink container spaced apart from the printhead.
Ejection of an ink drop through a nozzle employed in a thermal inkjet printer is accomplished by quickly heating a volume of ink within the adjacent ink firing chamber by applying an energizing electrical pulse to a heater resistor positioned in the ink firing chamber. The electrical pulse induces a temperature rise in the heater resistor, which heat energy is transferred to the ink to produce an ink vapor bubble. The rapid expansion of the ink vapor bubble forces ink through the nozzle. Once ink is ejected, the ink-firing chamber is refilled with ink from the ink channel and ink supply. The energy required to eject a drop of a given volume is referred to as turn-on energy. The turn-on energy is an amount of energy sufficient to form a vapor bubble having sufficient size to eject a predetermined amount of ink through the printhead nozzle.
The printhead includes a substrate, which is a conventional silicon wafer upon which has been grown a dielectric layer, such as silicon dioxide. The ink drops are ejected from small ink chambers carried on the substrate. The chambers (designated “firing chambers”) are formed in a component known as a barrier layer. The barrier layer is made from photosensitive material that is laminated onto the printhead substrate and then exposed, developed, and cured in a configuration that defines the firing chambers.
The heater resistor for ejecting a drop is a heat transducer, such as a thin-film resistor. The resistor is carried on the printhead substrate. The resistor is covered with suitable passivation and other layers and connected to conductive layers that transmit current pulses for heating the resistors. One resistor is located in each of the firing chambers.
In a typical printhead, the orifice plate covers most of the printhead. The orifice plate may be electroformed with nickel and coated with a precious metal for corrosion resistance. Alternatively, the orifice plate is made from a laser-ablated polyimide material. The orifice plate is bonded to the barrier layer and aligned so that each firing chamber is continuous with one of the orifices.
To refill the firing chambers after each drop is ejected, each chamber is continuous with an ink channel that is formed in the barrier layer. The channels extend toward an elongated ink feed slot that is formed through the substrate. The ink feed slot may be located in the center of the printhead with firing chambers located on opposite long sides of the feed slot. The slot is made after the ink-ejecting components (except for the orifice plate) are formed on the substrate.
The above-described components (barrier layer, resistors, etc) for ejecting the ink drops are mounted to the front side of the printhead substrate. The back side of the printhead is mounted to the body of the ink cartridge so that the ink slot is in fluid communication with an opening to the reservoir. Thus, refill ink flows through the ink feed slot from the back side of the substrate toward the front of the substrate and then across the front side through the channels (and beneath the orifice plate) to refill the chambers.
Significant effort has been expended in improving print quality. Since the image output of an inkjet printer is formed of individual ink drops, the image quality and contrasts, as well as variations in image hue and lightness, are dependent on ink drop volume and ink drop distribution on the printed medium. It is known that drop volumes vary with printhead substrate temperature because the properties that control them vary with temperature: the viscosity of the ink itself and the amount of ink vaporized by a heater resistor when driven by a given electrical printing pulse. One method of controlling drop volume is to vary the electrical pulse width supplied to the heater resistor. However, inkjet ink is chemically reactive, and prolonging exposure of the heater resistor and its electrical connections to the ink may result in a chemical attack upon the heater resistor and deterioration in the long-term performance of the heater resistor. Another method of controlling drop volume is to construct a protective layer having a thickness gradient over the heater resistor. However, varying the thickness of the protective layer is subject to the tolerances of the semiconductor manufacturing process and to the tolerances in the heat conduction gradients of the protective materials.